A tragic storyline isn't necessarily a dark one. For example, the theories some people have had about her having a miscarriage or losing a child that was very young would be a tragic and heavy storyline but not a particularly dark on. If, however, James were to have suffered a mental breakdown or been suffering some sort of PTSD and smothered their child to death, that would be dark.
This is why I feel there's a distinction between what is tragic and what is dark, and they can't be lumped in together like they're the same thing. Death, murder, rape, psychotic behaviour, etc, are what I would consider be dark themes and anything other than that is just tragic and heavy. The dark themes would prevent or at least dampen a happy ending, whereas a tragic one does not.